The present invention relates to a process for the production of yellow bis(3-[trialdoxysilyl]-alkyl)polysulfanes.
The production of bis(3-[triethoxysilyl]propyl)-tetrasulfanes by reacting chloropropyltriethoxysilane with sodium polysulfide (NPS) is known from DE 21 41 159.
The production of bis(3-[triethoxysilyl]propyl)disulfane by reacting chloropropyltriethoxysilane with sodium polysulfide (NPS) and sodium sulfide (Na2S) is known from DE 10034493.3. Alternatively, bis(3-[triethoxysilyl]-propyl)polysulfanes can be produced by reacting chloropropyltriethoxysilane with sodium sulfide (Na2S) and sulfur. The chloropropyltriethoxysilane used in the known processes can be produced by ethanolysis of chloropropyl-trichlorosilane. In this connection a fully reacted product with only a very small proportion of chloropropyl-monochlorodiethoxysilane is obtained according to known processes such as described in DE 20 61 189 and DE 32 36 628. This completely reacted chloropropyltriethoxysilane is hereinafter termed xe2x80x9cneutralxe2x80x9d.
If the neutral chloropropyltriethoxysilane is reacted with sodium polysulfide (NPS, Na2Sx where x=2 to 5 and mixtures thereof) or sodium sulfide (Na2S) and sulfur or sodium polysulfide (NPS, Na2Sx where x=2 to 5 and mixtures thereof) and Na2S for the production of bis(3-[triethoxysilyl]propyl)polysulfanes according to the processes specified above, then a dark yellow to red product (iodine color index xe2x89xa720 mg iodine/100 ml) is obtained. However, a pale yellow product has been introduced on the market that cannot be obtained from neutral chloropropyltriethoxysilane.
The disadvantage of these known processes is that a product having a dark yellow to red color is obtained.
In order to obtain pale yellow bis(3-[triethoxysilyl]-propyl)polysulfanes (iodine color index xe2x89xa610 mg iodine/100 ml) a so-called residual acid content in the form of chloropropylmonochlorodiethoxysilane must be present in the chloropropyltriethoxysilane. This can be achieved if the ethanolysis reaction is not carried out to completion. This procedure introduces a not inconsiderable additional complication in operational practice however, especially since the residual acid content has to be kept within a very narrow range, and the reaction therefore has to be discontinued in a very targeted manner at a specific point shortly before complete conversion.
Also, an acidification of the neutral chloropropyltri-ethoxysilane with alcoholic hydrochloric acid before the reaction with the aforementioned sulfurisation agents does not lead to pale yellow bis(3-[triethoxysilyl]propyl)-polysulfanes, but instead to dark yellow to red bis(3-[triethoxysilyl]propyl)polysulfanes.
The production of pale yellow bis(3-[triethoxysilyl]-propyl)polysulfanes by adding a small amount of chloropropyltrichlorosilane or chloropropylmonochloro-diethoxysilane or chloropropyldichloromonoethoxysilane to chloropropyltriethoxysilane before addition of the sulfurization agents is known from DE 100 24 037.2 and DE 100 45 269.8.
The disadvantage of this process is that chlorosilanes have to be handled, which on account of their properties are aggressive and highly corrosive compounds that release hydrogen chloride vapors in the presence of moisture.
An object of the present invention therefore is to provide an alternative process by means of which a yellow bis(3-[trialkoxysilyl]alkyl)polysulfane can be obtained and in which the additional use of aggressive chlorosilanes can be dispensed with.
The above and other objects of the invention can be achieved by a process for the production of yellow bis(3-[trialkoxysilyl]alkyl)polysulfanes with an iodine color index of xe2x89xa610 mg iodine/100 ml, preferably 5-7 mg iodine/100 ml, which is characterised in that an organic acid, for example formic acid, formyl chloride, acetyl chloride, acetic acid or mixtures of these acids are added to neutral chloroalkyltrialkoxysilane and then reacted with sodium polysulfide (NPS) or sodium sulfide (Na2S) and sulfur or sodium polysulfide (NPS) and Na2S in alcohol.
The sodium polysulfide (NPS) may be Na2Sx where x=2 to 5 or mixtures thereof.
In accordance with the present invention, preferably the bis(3-[trialkoxysilyl]alkyl)polysulfanes may be bis(3-[triethoxysilyl]propyl)polysulfane, bis(3-[triethoxysilyl]ethyl)polysulfane, bis(3-[triethoxysilyl]-methyl)polysulfane, bis(3-[trimethoxysilyl]propyl]-polysulfane, bis(3-[trimethoxysilyl]ethyl)polysulfane or bis(3-[trimethoxysilyl]methyl)polysulfane.
As chloroalkyltrialkoxysilane there can be used chloropropyltriethoxysilane, chloroethyltriethoxysilane, chloromethyltriethoxysilane, chloropropyltrimethoxysilane, chloroethyltrimethoxysilane or chloromethyltrimethoxy-silane. Chloropropyltriethoxysilane may particularly preferably be used.
The yellow bis(3-[trialkoxysilyl]alkyl)polysulfanes that may be produced according to the invention may be a polysulfane mixture. The polysulfane chain can on average contain 1.5-10.0 sulfur, preferably 2.0-4.0 sulfur.
The organic acid can be added in amounts of 0.1-10 wt. %, preferably 0.5-5.0 wt. %, particularly preferably 1.0-5.0 wt. %.
Propanol, ethanol or methanol can be used as alcohol. Preferably the alcohol derived from the alkoxy group may be used as alcohol, for example ethanol in the case of ethoxy groups. Lower alkanols; i.e. containing 1 to 5 carbon atoms are suitable.
The reaction mixture can be heated before addition of sodium polysulfide (NPS) or sodium sulfide (Na2S) and sulfur or sodium polysulfide (NPS) and Na2S, preferably at temperatures of 20-90xc2x0 C.
The organic acid, for example formic acid or acetic acid, can no longer be detected by means of NMR spectroscopy in the product, namely bis(3-[trialkoxysilyl]alkyl)polysulfane, after the working up, from which it may be assumed that no organic acid remains in the product.
Bis(3-[trialkoxysilyl]alkyl)polysulfanes produced by the process according to the invention have an iodine color index of xe2x89xa610 mg iodine/100 ml.
The process according to the invention has the advantage that the use of aggressive chlorosilanes can be avoided. The small amount of organic acid required for the production of yellow bis(3-[trialkoxysilyl]alkyl)-polysulfanes may be provided simply by adding for example formic acid or acetic acid to neutral chloropropyl-triethoxysilane. The use of organic acids has the advantage that no hydrogen chloride vapors are released in the presence of moisture.